Synthesis and Characterization of Germanium/Si−Alkyl and Germanium/Silica Core−Shell Quantum Dots

Ge/Si−R and Ge/SiO 2 core−shell nanoparticles have been synthesized by a solution route from the stepwise reactions of the Zintl salt, Mg 2 Ge, with SiCl 4 and subsequently either RLi (R = butyl) or H 2 O 2 . High-resolution transmission electron microscopy (HRTEM) images show that the core part of these Ge/Si−R and Ge/SiO 2 nanoparticles is crystalline and the d spacing of lattice fringes agrees with the {111} crystal plane (3.27 Å) of Ge. The average sizes (diameter) are 5.8(2.3) and 6.8(2.8) nm for Ge/Si−R and Ge/SiO 2, respectively. Fourier transform−infrared (FTIR) spectra are consistent with alkyl groups bonded to the silicon surface in Ge/Si−R nanoclusters. Off-axis holography was performed for several Ge/SiO 2 nanoparticles. The characterization of the ratio of radii and geometry between the core part and the outer sphere of Ge/SiO 2 is presented. The reconstructed phase image from the hologram offers detailed information concerning the ratio of the shell thickness ( R − r ) to the radius of the core ( r ), Rr = ( R − r )/ r, by measuring the phase shift and the resultant phase contrast. The Rr value from the deconvoluted phase image of a 16 nm spherelike Ge/SiO 2 is 0.264 (0.065). Photoluminescence (PL) spectra of both Ge/Si−R and Ge/SiO 2 nanoclusters show a strong UV−blue PL with a peak centered between 400 and 410 nm. These results suggest that the UV−blue PL may be attributed to the core part of these nanoparticles.